Ionization Energy

What is ionization energy?

The energy required to remove an electron from a neutral atom or cation in the gaseous phase.

What are I₁, I₂, and I₃ in ionization energy?

• I₁: First ionization energy — removing an electron from a neutral atom.

• I₂: Second ionization energy — removing an electron from a +1 cation.

• I₃: Third ionization energy — removing an electron from a +2 cation.

How do ionization energies change as electrons are removed (I₁ → I₂ → I₃)?

Each successive ionization energy is larger than the previous:

I1 < I2 < I3

This is because removing an electron from an increasingly positive ion requires more energy

What does a higher ionization energy indicate?

A stronger attraction between the nucleus and the electron, meaning more energy is needed to remove that electron.

What is the general trend for ionization energy across a period?

Ionization energy increases as you move across a period because Z* increases, strengthening the nuclear pull on electrons.

What are key exceptions to the ionization energy trend across a period?

Ionization energy is lower than expected when:

• Removing an electron empties a p-subshell.

• Removing an electron results in a half-filled p- or d-subshell, which is relatively stable.

• For d- and f-block elements, IE increases more gradually since d- and f-electrons are weakly penetrating and experience lower Z*.

What is the trend for ionization energy when moving from one period to the next (down a group)?

• Ionization energy drops significantly at the start of a new period.

• This is because removing an electron from the new outer s-subshell requires less energy; the electron is farther from the nucleus and more shielded.